Compositionally Sequenced Interfacial Layers for High‐Energy Li‐Metal Batteries

Abstract Electrolyte additives with multiple functions enable the interfacial engineering of Li‐metal batteries (LMBs). Owing to their unique reduction behavior, additives exhibit a high potential for electrode surface modification that increases the reversibility of Li‐metal anodes by enabling the development of a hierarchical solid electrolyte interphase (SEI). This study confirms that an adequately designed SEI facilitates the homogeneous supply of Li+, nonlocalized Li deposition, and low electrolyte degradation in LMBs while enduring the volume fluctuation of Li‐metal anodes on cycling. An in‐depth analysis of interfacial engineering mechanisms reveals that multilayered SEI structures comprising mechanically robust LiF‐rich species, electron‐rich P–O species, and elastic polymeric species enabled the stable charge and discharge of LMBs. The polymeric outer SEI layer in the as‐fabricated multilayered SEI could accommodate the volume fluctuation of Li‐metal anodes, significantly enhancing the cycling stability Li||LiNi0.8Co0.1Mn0.1O2 full cells with an electrolyte amount of 3.6 g Ah−1 and an areal capacity of 3.2 mAh cm−2. Therefore, this study confirms the ability of interfacial layers formed by electrolyte additives and fluorinated solvents to advance the performance of LMBs and can open new frontiers in the fabrication of high‐performance LMBs through electrolyte‐formulation engineering.


Characterization
Inductively coupled plasma-optical emission Spectroscopy (ICP-OES, 700-ES, Varian) was used to investigate the role of the CEI layer formed on the NCM811 cathode in different electrolytes on transition-metal (Ni, Mn, and Co) and Al dissolution from the electrode.The fully charged NCM811 cathodes were disassembled, washed with the solvent DME to remove any residual electrolyte, and stored in an oven at 60 ℃ for 3 days.The structures of the NCM811 cathodes cycled in the additive-free, LiPO 2 F 2 + LiNO 3 , and LiPO 2 F 2 + LiNO 3 + VC electrolytes were analyzed by high-resolution thin-film XRD (SmartLab, RIGAKU) using theta(θ)/2theta analysis within 10-80° at a scan rate of 1° min −1 .In this study, 2016-cointype 700-μm Li||Al cells were used for linear sweep voltammetry (LSV) at a scan rate of 1 mV s −1 at 25 ℃.

Computational details
The DMol 3 software was used for all DFT calculations to investigate the HOMO and LUMO energy levels, formation energies, bond-dissociation energies, and reaction mechanisms.
Beck's three-parameter hybrid functional combined with the Lee-Yang-Parr correlation functional was used for electron exchange-correlation energy calculations.The effective core potential was used for core treatment with the double numerical plus polarization 4.4 level basis set.The Tkatchenko-Scheffler van der Waals correction method was used for spin-polarized calculations.The convergence criterion for self-consistent calculations was set as 1 × 10 -6 Ha, and the convergence criteria for geometry optimization were set as 1 × 10 -5 Ha, 0.002 Ha Å -1 , and 0.005 Å for the maximum energy change, maximum force, and maximum displacement, respectively.The organic-solvent environment for analysis was implicitly constructed by the conductor-like screening model using the dielectric constant of 1,2-dimethoxy ethane (DME) (7.2).The reaction-mechanism transition states were calculated using the generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-correlation functional.The complete single linear synchronous transit and quadratic synchronous transit methods were used for calculations considering 0.002 Ha Å - 1 to be the root mean square convergence force on the atoms.5

Figure S1 .
Figure S1.a) Raman spectra of the different electrolytes and b) proportion of solvation

Figure S2 .
Figure S2.Schematic illustration of the solvation structures of the additive-free (our system)

Figure S3 .
Figure S3.Schematic illustration showing different wetting states of the cathodes

Figure S4 .
Figure S4.Energy-level diagram showing the HOMO and LUMO of the solvents (DME,

Figure S7 .
Figure S7.LUMO energy levels of TFOFE during one-, and two-electron reduction.The

Figure
Figure S9.a) Adsorption sites of Li + on C 2 HOF 4 − , marked by colored-dash boxes.b)

Figure S11 .
Figure S11.(a) 19 F NMR spectra of the TFOFE solvent before and after storage with Li-

Figure S12 .
Figure S12.Electrochemical reduction behaviors of 2.5 M LiFSI DME and 2.5 M LiFSI DME/TFOFE (8/2 vol%) on the Li-metal of the Li||Cu cells; the results are measured at a scan rate of 0.1 mV/s during the initial Li plating.UPD represents "under potential decomposition."

Figure S14 .
Figure S14.Comparison of the oxidation stability of the additive-free electrolyte and DME

Figure S16 .
Figure S16.TOF-SIMS depth profile of the multilayered SEI on the Li-metal anode,

Figure S18 .
Figure S18.Atomic percentages of different elements in the a) inner and b) outer SEI layers

Figure S23 .
Figure S23.a) Elastic modulus and thickness of the SEI layer formed in the LiPO 2 F 2 + VC

Figure S24 .
Figure S24.The excluded force-indentation depth curves of Li-metal anodes formed in the

Figure S25 .
Figure S25.The excluded force-indentation depth curves of Li-metal anodes after 1 cycle

Figure S27 .
Figure S27.Cross-sectional and surface SEM images of Li-metal anodes extracted from

Figure S29 .
Figure S29.Voltage profiles of initial Li plating/stripping in Li||Cu cells containing the

Figure
Figure S30.a) Voltage curves for initial Li plating and stripping of the Li||Cu cells with

Figure S32 .
Figure S32.Cross-sectional and surface SEM images of an Li-metal electrode extracted

Figure S33 .
Figure S33.Surface SEM images of an Li-metal electrode extracted from Li||Cu cells

Figure S34 .
Figure S34.LSV of electrolytes with a stainless-steel working electrode at a scan rate of 1

Figure S36 .
Figure S36.Cycling performance of Li||NCM811 full cells containing electrolytes with

Figure S43 .
Figure S43.Cross-sectional SEM image of an Li-metal anode extracted from an Li||NCM811

Figure S44 .
Figure S44.ICP-OES analysis of Ni and Al dissolution from NCM811 cathodes extracted

Table S1 .
Ionic conductivities and densities of the electrolytes